Switch to Metric? Yes or No

[Keith Outten]

Jerome, your right about my laser engraver and CNC router both are capable of either imperial or the metric system. However they are simple software configurations so there's no reason to be concerned with making a mistake. Even if I load a metric drawing the software automatically converts it to imperial dimensions, but that's unlikely to happen since I don't accept metric projects. I am fortunate that I am in a position to be able to bid or no-bid projects that don't fit my requirements. Currently I am basically semi-retired so I have even more flexibility concerning my shop schedule.

I can't remember any of the details but many years ago NASA had an incident where a metric dimension was misinterpreted as imperial and the lander missed a planet by a hundred miles. They were embarrassed about the incident but it was just taxpayers money so no big deal. If that happened to me in my little shop it would be devastating.

I spent half my life as a Quality Control Inspector and I have had my share of situations where a machine shop I was at to inspect a project before they could ship and found dimensional errors or even drawings that were not the current revisions. I've seen shop supervisors faces turn white as a ghost when I explained a problem I found and they instantly knew that they were in big financial trouble. I learned early in life how serious errors can be and when I opened my own shop I made certain decisions about basic policies I had to abide by to limit my own liability. Inspectors make a living based on the fact that people make mistakes, we had T-Shirts at North Anna Power Station in the late 70's that said "We Live By Your Mistakes". Sometimes they were billion dollar mistakes that made it crystal clear to the customer that inspection was worth every penny invested

There's nothing simple about any system of measurement, its more like a science than it is a directive in a specification. For instance there is a right way and a wrong way to use a simple micrometer, if you want to learn the right way find a Master Machinist and ask him to teach you how its done properly.

It really is about the money folks
.

[Mark Wooden]

Hmmmm....
I started to read all the replies here and lost interest in the debate and decided to post my opinion, which, with $2, will get you a medium cup of coffee.....
I tend to side with Martin Wasner, doesn't matter what the system is, the physical universe rules in measurements. Worked with a guy once who used actual sticks marked out on site for measurements, rarely used a rule. Very accurate.

But, in answer to the OP-
Start with Metric , Finish with Metric. Same for Imperial. Don't do conversions, waste of time. Don't use combo rules either- too easy to get confused and make a mistake. If I have to use a lot of components that are metric sized, I'll tend to do the entire job in metric and I put the imperial rules and tapes away.
Use whatever system is easiest for you, but my advice is to not mix them. YMMV

[Jim Becker]

Start with Metric , Finish with Metric. Same for Imperial. Don't do conversions, waste of time.

Sage words....

[Curt Harms]

Strange as it sounds, I think the hardest part of switching from imperial to metric is visualizing the measurements. Imperial is native- it’s like thinking in English.

Not to a young person growing up in a country that has been metric for longer than they've been alive.

[Rob Luter]

Decimal inch for me. I think in three place decimals.

[Brian Holcombe]

I have metric, shaku and imperial tools and if one fixates to much on the dimensional size of them, it will drive you crazy. Another added to the group recently since I have a slot mortiser and the good bits are in metric, 6mm is damned close to 1/4"....but not exactly. It's off by 14 thousands, thats enough to make a pretty unsightly gap in a setup. I counter this by always setting my marking gauges to the tool, not to the measurement.

I've grown accustomed to doing this for Japanese tools which are sold in metric but they're made in Shaku (traditional Japanese measurement system). Shaku is very close to inches, so they end up measuring almost imperial and almost metric.

My friend Jim, a wonderfully talented teahouse carpenter, described Shaku as being just close enough to screw up your work. One Shaku is 11.93", almost enough to ignore but not quite. One Bu is .1193", very close to 1/8" and two Bu is .238", right there between 1/4" and 6mm.

So rather than drive myself nuts, I simply set my gauges or tool setting exactly to the work or the tool bit, never to the measurement.

[Dave Zellers]

So rather than drive myself nuts, I simply set my gauges or tool setting exactly to the work or the tool bit, never to the measurement.

Great tip.
Rather similar to cabinet makers who don't use tape measures but rather transfer measurements with a story stick.

[Dan Schmidt]

For my type of woodworking projects, 1/16" never felt accurate enough. 1/32" was overkill and harder to read/assess. I found 1mm (which is in between) to be perfect. All of my equipment and scales are set up for dual measurement scales, but I find myself only using metric. Took a bit of getting used to but now I would never go back.

Dan

[Phillip Gregory]

I use imperial measurements as tooling, stock, and most woodworking fasteners widely and easily available in the US are imperial. The only metric in my shop is fasteners on the newer equipment.

I've run into all kinds of stuff in construction that's "metric" but when you convert you find that it's based on an imperial standard. Not just construction either. If you know anything about firearms you've heard of the AK47 round which is 7.62x39. That's a bullet diameter of 7.62 mm. Guess what that converts to. 0.300", which is "30 caliber".

The 7.62x39 uses the 0.311" bullet from the .303 British. It uses (or should use) a different bullet than the American 7.62x54 which despite being a "7.62 mm" round uses 0.308" bullets. Shooting a 7.62x39 round loaded with a .308 dia bullet through an AK leads to poorer than normal accuracy, shooting a 7.62x54 round loaded with a .311 dia 7.62x39 bullet may blow up the rifle.

9mm or .45 ACP?

A .45 ACP bullet is close to 12 mm in diameter, a 9 mm bullet is just a fuzz smaller than a .38/.357 so they are quite a bit different in size. Personally, I'd prefer neither. Both of those are rimless cartridges that headspace on the case mouth. Give me a rimmed cartridge any day, way easier to reload and easier to shoot in a revolver or pistol caliber lever gun. I'm not a big fan of semi-autos.

[Ted Derryberry]

I wasn't implying that the bullets were actually the same. Anybody that knows anything about firearms knows that few cartridges or bullets are the actual size of their designation. Similar to "2x4s". My point is that common metric cartridges, like many "metric" standards in other industries, are often based on imperial units.

7.62mm does in fact convert to exactly 0.300".

[William Adams]

For what it's worth, my understanding is that 38 special isn't taken from a decimal designation, but rather a corruption of the 3/8" drill which was used to enlarge the cylinder of a cap and ball revolver when converting it from black powder, which is why the actual bullet dimension is .357.

[Phillip Gregory]

For what it's worth, my understanding is that 38 special isn't taken from a decimal designation, but rather a corruption of the 3/8" drill which was used to enlarge the cylinder of a cap and ball revolver when converting it from black powder, which is why the actual bullet dimension is .357.

The designation came from the earliest days of metallic cartridges when all metallic cartridges were rimfire with heeled bullets the same diameter as the case, like the .22 Long Rifle. The .38 Special's case diameter is 0.379" which is essentially identical to the old .38 caliber rimfire rounds which actually did use about an 0.38" diameter bullet. The .38 Special being a more modern centerfire cartridge uses non-heeled bullets that have to fit inside the case mouth and thus must be smaller in diameter.

[johnny means]

...but my advice is to not mix them. YMMV

So true. i was having a conversation at work about making a part. Coworker didn't seem to be catching my drift no matter how many times i ran through it. finally, he looks at me and asks,"Are you talking standard or metric?" It took me a second to realize that I'd been switching back and forth for an entire ten minute conversation about measurements. I'm not some sort of conversion savant, but i do tend to see most specs in standard, while hardware mounting comes to me in metric.

[Jim Becker]

Not mixing is a good idea...human nature can be dangerous in that situation. Related to that, I started a new project yesterday which had been planned for some time and was originally sketched out and cut-listed in inches. It didn't take long for it to drive me nuts because I've thoroughly been enjoying working in metric for a few months now...so much so, that I left the shop, remeasured the job (this one is for my house so I didn't have to go far ), hit the computer to update the cut-list and then went out and cut out everything. Even when I was doing the "figuring" for a planned mini-split, the tape with inches didn't last long and I just switched out to the metric tape since the diagrams had both measurements. I guess my mind has gone far enough in that direction that it's now compelling...but I'm weird like that.

[Chris Parks]

You just validated my comment on the US changing by degrees or erosion of imperial as people and industry work out that it is a better system overall.